Electric lamps

ABSTRACT

A lamp, particularly applicable as a vehicle headlamp, comprises a point source light at the focus of an ellipsoidal reflector. The beam is brought to a sharp focus outside the lamp at the second focus of the ellipsoid and a small screen, at or near this point will give a sharp cut-off to the beam or a sharply defined shadow. The ellipsoidal reflector is preferably formed by the envelope of a lamp having a tungsten ball as the light source.

United Mates ?atellt 1191 1 1 l w Giller [451Marc11 2Q, 1973 1 ELECTRIC LAMPS R17,037 7/1928 [76] Inventor: William Leslie Giller, The Spinning, if

Gravelly H111, Caterham, England 21453: 1 1 1/194 2,667,592 1/1954 [22] 1969 2,260,736 10 1941 Benton ..313 110 [2]] Appl. No.: 873,982

FOREIGN PATENTS OR APPLICATIONS Foreign Application Priority Data 1,01 1,374 7/1957 Germany ..240/46.53

July 2, 1969 Great Britain 33,378/69 Primary Examinerl-lerman Karl Saalbach Assistant Examiner-Wm. H. Punter [52] US. Cl. ..3l3/I14, 313/113, 313/215, AttorneyBeveridge & De Grandi 313/216, 240/4653 [51] Int. Cl.....H01k 17/06, HOlk 17/10, F21v 11/16 [57] ABSTRACT Fleld of Search 10, 46, 39, A p particularly applicable as a vehicle headlamp,

313/185 1 315/76; 240/81 comprises a point source light at the focus of an ellipsoidal reflector. The beam is brought to a sharp focus [56] References C'ted outside the lamp at the second focus of the ellipsoid and a small screen, at or near this point will give a UNITED STATES PATENTS sharp cut-off to the beam or a sharply defined shadow. 2,277,685 3/1942 Bergstrom ..240/46.55 X The ellipsoidal reflector is preferably formed by the 3,373,713 4/1968 Ludwig .-3 /1 3 envelope of a lamp having a tungsten ball as the light 3,253,504 5/1966 Vollmar ..88/24 Source, 3,257,574 6/1966 McLintic ..3l3/l 13 1,822,360 8/1931 Miesse ..313/2l5 9 Claims, 2 Drawing Figures PATEmEnuARzolma 3,721,850

HORIZONTAL WIRE 0R SCREEN 44 INVENTOR WILLIAM LESks GILLER ELECTRIC LAMPS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to lamps, particularly but not exclusively to lamps for road vehicles.

2. Description of the Prior Art The present common practice in producing a beam of light, e.g. for a headlamp for a motor vehicle, is to use a tungsten filament bulb mounted in a lantern having a reflecting surface behind the bulb and a window or lens in front of the bulb. To an external observer, the whole of the window or lens forms a bright light source. Because of the finite dimensions of the bulb filament, whatever the shape of the reflecting surface or the form of the lens, every point in the lens or window is radiating light in a multiplicity of directions. A large part of the light is wasted by not being beamed in a required direction and moreover is wasted in a manner which, in many circumstances causes glare and inconvenience. The glare problem cannot be overcome by masking or beam shaping because, due to the multiplicity of directions of the light from a large effective light source, it is not possible to produce a sharp cut-off to the beam or a sharply defined shadow region.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved form of lamp giving light radiation in which the individual rays of light are much more closely controlled to be in the required directions, in other words the beam of light is much purer as the individual rays (except where deliberately brought to a focus) do not intersect or interfere with one another.

Considered in its broadest aspect, a lamp according to the invention comprises a point source of light in combination with means focussing the light from said source at a point in front of the lamp.

More particularly the invention includes a lamp comprising an envelope with an ellipsoidal reflecting surface and a transparent window and having a point source of light within said envelope at a focus of the ellipsoidal surface.

In a preferred form, an electric lamp comprises a sealed envelope containing a small tungsten ball to form a point source, the tungsten ball being arranged as an anode, the envelope further containing a cathode arranged adjacent the ball so that an arc may be struck between the cathode and anode and having the envelope, which contains argon and/or nitrogen or any other suitable gas, shaped so that a part thereof coated with reflecting material forms an optical reflecting surface to focus light forwardly from the ball through a window in the envelope.

The tungsten ball and cathode may be arranged as in the form of a lamp known as a Pointolite lamp (see Discharge lamps for photographic projection by l-I.I(. Bourne, Chapman and Hall). In this lamp an arc is struck between a cathode and a tungsten ball forming an anode, a tungsten spiral being used to initiate the arc.

The tungsten ball forms an intense light source of very small size and hence it is possible to direct the light accurately by reflection from a shaped reflecting surface with the ball at the focus of the reflecting surface. The window may be a flat surface at right angles to the direction of the beam (the major axis of the ellipsoid).

0 angular width.

By the use of a reflecting surface and point source as described above, it is readily possible to direct more than half the total light output, by reflection from the reflecting surface, through the window as rays in controlled directions. To avoid interference with this reflected light, all light rays direct from the tungsten ball through the window may be suppressed by forming the cathode as a small shield, close to the tungsten ball, which is of just sufficient size to intercept direct radiation from the ball to the window.

The whole envelope or substantially the whole envelope may be made of glass which is coated with reflecting material over the region where a reflecting surface is required. The envelope is shaped to form the required reflecting surface and is preferably formed with a flat window across the region where light is to be emitted. The window may be integral with the reflecting surface or the window may be sealed to the reflecting surface.

Instead of using a shaped envelope as the reflecting surface, the lamp may be formed as a pointolite lamp in a small bulb which is mounted with the ball at the focus of a separate ellipsoidal reflector.

Because the light emitting source is very small compared with conventional filament lamps, the opening in the glass envelope required for insertion of the anodecathode assembly is much smaller than that required for filament lamps and may be little more than is necessary for the insertion of the evacuation tube and sealed leads. Thus this part of the envelope causes very little interference with the production of the required light beam. The location of the point in the envelope where the leads are taken through the envelope will depend on the use of the lamp. In some cases, with ellipsoidal reflecting surfaces forming part of the envelope, it may be most convenient to make the window orthogonal to the major axis and to have the lead assembly extending through the ellipsoidal surface on this major axis. In other cases, it may be preferred to take the leads through the envelope near the junction between the window and the reflecting surface.

The envelope may be made very large compared with the ball and may thus form a completely sealed unit giving a directional light beam. Such a lamp gives a much stronger source of light for the same size of assembly than is possible with systems using tungsten filament bulbs arranged in a separate reflector.

The lamp of the present invention finds particular application as a headlamp for road vehicles. By using the lamp of the present invention, it is possible to provide an improved form of lighting system for use on road vehicles which, while still effectively illuminating the road ahead, will reduce the glare for oncoming drivers and which can also be arranged to give drivers of oncoming vehicles a clear vision of the vehicle fitted with the improved lighting system.

According to a further feature of the invention, a lighting system for road vehicles may comprise one or more lamps, as described above, mounted on the vehicle, and for each lamp, a screen positioned forwardly of the lamp, the lamp being arranged to give a beam focussed substantially at the position of the screen and the screen being positioned to cast a shadow (black beam) at a level covering the expected range of heights above the road surface of the eyes of drivers of oncoming vehicles.

With the lamp of the present invention, the beam of light is sharply concentrated and will contract in cross sectional area as it leaves the lamp to a very small focus. A small screen in the region of the focus can give a sharply defined shadow or black beam. With correct focussing, it would be possible to obtain total eclipse of the beam by a screen of say 3'4 inch diameter. In particular it is possible to obtain a sharply defined shadow of any shape or size so that the road is illuminated in front of the vehicle but the light does not shine directly on the eyes of drivers of oncoming vehicles.

Preferably the lamp or each lamp is mounted on the vehicle in a position at or near the level of the drivers head. It may be behind the driver and the screen may be positioned forwardly of the driver and may typically be several feet in front of the lamp.

When vehicles pass on opposite sides of the road, the glare problem arises only when the vehicles are an appreciable distance apart since, as they get very close, the angular direction of the light beam and the narrowness of the beam in the horizontal plane should avoid dazzling the driver of the oncoming vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a lamp assembly; and

FIG. 2 is a diagram illustrating how the lamp of FIG. I can be used to give a sharply defined shadow or cutoff to the beam.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 there is shown an electric lamp consisting of a glass envelope 20 with a front window 21 which conveniently is flat or slightly curved. The rear part of the envelope 22 is ellipsoidal in form. In longitudinal section it is elliptical but in transverse section it is circular. The window is in a plane generally normal to the major axis of the ellipsoid. One focus of the ellipsoid is inside the envelope and, at this focus, a tungsten ball 23 forms an anode. This ball is supported on a conductor 24 which, in this particular construc' tion, extends rearwardly to pass through the envelope 20 with a glass seal 25. Immediately in front of the ball 23 is a cathode 26 on a conductor 27. In use an arc is struck between the cathode 26 and the ball 23, the ball becoming white hot to form a light source of very small dimensions. The are is initiated using a tungsten spiral 28 on a conductor 29. All three conductors 24, 27, 29 in this construction extend through a single seal 30 in the envelope 20. The bulb is filled with a mixture of argon and nitrogen or other suitable gas.

The inner or the outer surface of the ellipsoidal rear part 22 of the envelope 20 is coated with a light reflecting material 31. Because the ball 23 forming the point source is at the focus of this ellipsoidal surface, all the light from the ball, after reflection from the reflecting surface 31, passes out through the window 21 and converges on the second focal point of the ellipsoid. By this arrangement, the light rays are brought to a very sharp focus at a position remote from the lamp. The cathode 26 is shaped so as to intercept all or at least a major part of the direct rays from the ball 23 towards the window 21.

FIG. 2 illustrates how the lamp of FIG. 1 can be used to give a sharp cut-off to the beam or to give a black beam, i.e. a complete shadow within the light beam. In order to show this, the angular spread of the beam has had to be greatly exaggerated and the forward focus has been brought much nearer the lamp than would be the case in practice. In FIG. 2 a lamp 40 produces light radiation convergent on a forward focus 41. A small screen 42 near this focus 41 will produce a sharply defined shadow 43 indicated by cross-hatching within a beam indicated by the outermost rays 44.

For use as a vehicle headlamp, the lamp may be arranged to produce a beam diverging from the forward focus (where the size of the beam might be say 0.75 inches diameter as an absolute point focus in practice is not possible) to enlarge to a circle of about 30 ft diameter at adistance of 300 feet. The screen 42 might consist of a horizontal wire about A inch diameter close to the focus to produce a black shadow at the level of the eyes of oncoming drivers. This shadow may be only 4 feet deep at a distance of 300 feet. The beam below the shadow illuminates the road.

I claim:

1. A lamp comprising an envelope with an ellipsoidal reflecting surface and a transparent window and having a point source of light within said envelope at a focus of the ellipsoidal surface, wherein said point source of light comprises a tungsten ball with a lead for connection to one terminal of an electric power supply, said ball forming an anode of an are system, a cathode with a lead for connection to a second terminal of said supply, the anode and cathode being arranged so that an arc can strike between the anode and the cathode causing the tungsten ball to become a light source, said anode and cathode leads passing through the reflecting surface along the major axis of the ellipsoid.

2. A lamp as claimed in claim 1 wherein said envelope is glass and wherein said ellipsoidal reflecting surface is formed by a portion of said envelope coated to form a reflector.

3. A lamp as claimed in claim 2 wherein the window is formed by a substantially flat portion of the glass envelope lying in a plane normal to the major axis of the ellipsoid.

4. A lamp as claimed in claim 1 wherein said window is integral with the reflecting surface.

5. A lamp as claimed in claim 1 wherein said window is sealed to the reflecting surface.

6. A lamp as claimed in claim 1 wherein said window is formed by a substantially flat portion of glass lying in a plane normal to the major axis of the ellipsoid.

7. A lamp comprising:

a. an envelope having a transparent window portion, at least part of said envelope having an ellipsoidal reflecting inner surface, said ellipsoidal surface having a first focus inside said envelope and a second focus outside said envelope,

b. a point source of light at said first focus, said point source comprising a tungsten ball with a lead for connection to one terminal of an electric power supply, said ball forming an anode of an are system and a cathode with a lead for connection to a second terminal of said supply, the anode and cathode being arranged so that an arc can strike between the anode and cathode causing the tungsten ball to become a light source,

said cathode being formed as a shield to intercept direct radiation from the ball to the window.

8. A lamp comprising an envelope having a transparent window portion, at least part of said envelope having an ellipsoidal reflecting inner surface, said ellipsoidal surface having a first focus inside the envelope and a second focus outside said envelope, a point source of light at said first focus and means forming a shield between said first focus and said window to intercept direct radiation from the point source of light to the window, in combination with a horizontal wire positioned near said second focus to partially intercept a light beam from said point source of light, and thereby produce a horizontally extending shadow area extending through the beam.

9. A lamp comprising an envelope having a transparent window portion, at least a part of said envelope having an ellipsoidal reflecting inner surface, said ellipsoidal surface having a first focus inside said envelope and a second focus outside said envelope, and a point source of light at said first focus, in combination with a horizontal wire positioned near said second focus to partially intercept a light beam from said point source of light and thereby produce a shaped shadow. 

1. A lamp comprising an envelope with an ellipsoidal reflecting surface and a transparent window and having a point source of light within said envelope at a focus of the ellipsoidal surface, wherein said point source of light comprises a tungsten ball with a lead for connection to one terminal of an electric power supply, said ball forming an anode of an arc system, a cathode with a lead for connection to a second terminal of said supply, the anode and cathode being arranged so that an arc can strike between the anode and the cathode causing the tungsten ball to become a light source, said anode and cathode leads passing through the reflecting surface along the major axis of the ellipsoid.
 2. A lamp as claimed in claim 1 wherein said envelope is glass and wherein said ellipsoidal reflecting surface is formed by a portion of said envelope coated to form a reflector.
 3. A lamp as claimed in claim 2 wherein the window is formed by a substantially flat portion of the glass envelope lying in a plane normal to the major axis of the ellipsoid.
 4. A lamp as claimed in claim 1 wherein said window is integral with the reflecting surface.
 5. A lamp as claimed in claim 1 wherein said window is sealed to the reflecting surface.
 6. A lamp as claimed in claim 1 wherein said window is formed by a substantially flat portion of glass lying in a plane normal to the major axis of the ellipsoid.
 7. A lamp comprising: a. an envelope having a transparent window portion, at least part of said envelope having an ellipsoidal reflecting inner surface, said ellipsoidal surface having a first focus inside said envelope and a second focus outside said envelope, b. a point source of light at said first focus, said point source comprising a tungsten ball with a lead for connection to one terminal of an electric power supply, said ball forming an anode of an arc system and a cathode with a lead for connection to a second terminal of said supply, the anode and cathode being arranged so that an arc can strike between the anode and cathode causing the tungsten ball to become a light source, c. said cathode being formed as a shield to intercept direct radiation from the ball to the window.
 8. A lamp comprising an envelope having a transparent window portion, at least part of said envelope having an ellipsoidal reflecting inner surface, said ellipsoidal surface having a first focus inside the envelope and a second focus outside said envelope, a point source of light at said first focus and means forming a shield between said first focus and said window to intercept direct radiation from the point source of light to the window, in combination with a horizontal wire positioned near said second focus to partially intercept a light beam from said point source of light, and thereby produce a horizontally extending shadow area extending through the beam.
 9. A lamp comprising an envelope having a transparent window portion, at least a part of said envelope having an ellipsoidal reflecting inner surface, said ellipsoidal surface having a first focus insiDe said envelope and a second focus outside said envelope, and a point source of light at said first focus, in combination with a horizontal wire positioned near said second focus to partially intercept a light beam from said point source of light and thereby produce a shaped shadow. 